1. Field of the Invention
This invention relates to a process for the production of a molded article having positive temperature coefficient characteristics, and specifically, to a process for the production of a molded article having characteristics that its resistance value (resistivity) increases with an increase in temperature, i.e. positive temperature coefficient (PTC) characteristics.
More specifically, this invention relates to a process for the efficient production of a molded article having positive temperature coefficient characteristics usable as a heating element, overcurrent protective element, or the like for a variety of uses.
2. Prior Art
In general, a molded article produced from a resin composition imparted with suitable electrical conductivity (suitable resistance value) by incorporating electrically conductive particles such as carbon black particles, etc., into a crystalline resin has positive temperature coefficient (PTC) characteristics, i.e. a positive coefficient for an increase in resistance value with an increase in temperature. Such a molded article is used in various fields including a heating element, an overcurrent protective element, a temperature protective element, and the like.
A molded article having positive temperature coefficient characteristics, obtained from such a resin composition, is particularly useful, since it has excellent formability or moldability, lightweight properties, flexing characteristics or flexibility over a conventional molded article having positive temperature coefficient characteristics, obtained from an inorganic compound.
Such a conventional molded article having positive temperature coefficient characteristics is produced by a process disclosed in JP-A-62-232903, which comprises kneading a mixture of a crystalline resin, electrically conductive particles (carbon black particles in particular) and organic peroxide at a temperature of not lower than the melting point of the above crystalline resin and at a temperature of not higher than the one minute half-value period temperature of the above organic peroxide; pressing the resultant kneaded mixture into a sheet at a temperature of the above half-value period temperature; and thereafter crosslinking the above crystalline resin. There is another process disclosed in JP-A-62-232902, which comprises crosslinking the above crystalline resin in an inert gas atmosphere; pulverizing the crosslinked product; mixing it with rubber or the like: and molding the resultant mixture to obtain a molded article.
However, the above processes have the following problem; Since the crosslinking treatment is carried out at a temperature of the one minute half-value period temperature of the organic peroxide and under such moderate conditions that no shear force is exerted on the kneaded mixture, the resultant molded article has a low rate of an increase in resistance value with an increase in temperature (insufficient PTC characteristics). The above processes also have a problem in that the operation of the (press)-molding is complicated and the continuous production is very difficult.
Meanwhile, there is another process which comprises adding a crosslinking agent while a polymer and electrically conductive particles are kneaded; crosslinking the polymer while the resultant mixture is kneaded to prepare pellets of a crosslinked kneaded mixture; and then, molding the pellets into an article (see JP-A-63-181401).
However, the above process has the following problems; The kneaded mixture to be molded is a composition whose crosslinkage has been already completed, and it is hence difficult to melt. Therefore, the above kneaded mixture is hard to mold. Further, when it is molded by applying a shear force to excess, the resultant molded article sometimes shows too high an initial resistance value (low-temperature resistance value) or a low rate of an increase in resistance value with an increase in temperature (PTC characteristics) due to rebreakage of its crosslinked chains.